The house is connected to a pipe network leading to a main sewer and centralised treatment works. The sewerage system includes a network of pipes, manholes, and inspection chambers, with larger sewer pipes carrying incoming sewage from smaller household pipes to the treatment facility.
The building has its own private pipes and treatment system, known as off-mains drainage and sewage treatment. In both cases, raw sewage enters the treatment process and must pass through several stages to become safe for discharge.
Proper sewage treatment is essential to protect water supplies and prevent contamination of drinking water, which is vital for public health. Owners of a property with off-mains drainage have two options:
Pump to the nearest main sewer if it is close enough to the property and according to building regulations requirements.
Install private sewage treatment in the form of a package sewage treatment plant or a septic tank.
A package sewage treatment plant is a small-scale wastewater processing unit that treats sewage through several main stages: primary, secondary, and tertiary treatment. This treatment process is designed to remove contaminants and produce treated sewage that is safe for discharge or reuse, protecting water quality, aquatic life, and public health.
In the first chamber, which serves as the primary treatment stage, sewage enters primary sedimentation tanks or large settlement tanks. Here, heavier solids sink to the bottom (heavier solids sink) and form sludge, while floating debris forms scum. Primary treatment removes these settleable solids and floating debris, preparing the liquid for further treatment.
The liquid then travels into a second chamber, which is the secondary treatment or biological stage. In this stage, biological treatment occurs using aerobic bacteria and processes such as the activated sludge method. These bacteria digest and remove remaining organic matter from the sewage. During this process, dead bacteria and more sludge are produced, which settle out in the final settlement tank, further clarifying the water.
Systems with a third chamber provide tertiary treatment, which may include additional processes to remove remaining particles and further improve effluent quality, such as disinfection by chlorination or ultraviolet light. The final effluent, or cleaned wastewater, is then safe for discharge to a drainage field or watercourse. Effluent quality is monitored using measures such as biological oxygen demand (BOD) to ensure environmental standards are met.
A septic tank is an underground storage container that collects wastewater flowing from a property and uses gravity to separate the water from the solids within the tank.
Septic tanks primarily provide primary treatment by separating human waste and solid waste from the liquid. The solids sink to the bottom of the tank, while the liquid flows out nearer the top, and soaks into the ground. Septic tanks require further treatment of the liquid effluent before it can be safely discharged to the environment.
Which planning rules and regulations apply?
As a householder, you are responsible for a septic tank or sewage treatment plant where your property uses an off-mains system. Depending on whether you select a package sewage treatment, or a septic tank, different planning rules and environmental regulations apply.
The Environment Agency is responsible for issuing permits, setting standards, and overseeing sewage treatment works to ensure environmental compliance and protection across the UK. Facilities managers and health & safety officers must ensure that sewage treatment plants comply with current legislation, including the General Binding Rules 2020, and any other relevant regulations. Monitoring and maintenance are critical to meet these standards, as the Environment Agency controls the amount of chemicals that can be discharged into nature and requires ongoing compliance for both domestic and commercial sewage treatment plants.
In England, you must make sure your system meets the Government’s general binding rules for small sewage treatment plants and septic tanks. Different rules apply in Wales, Scotland and Northern Ireland.
Read more – New domestic sewage treatment plants – 2023 rule updates.
Tank discharge outlet must be at least 10m from any watercourse or building
Must meet the EN12566-3 standard for small wastewater treatment systems
If the plant needs electricity to operate, it should be able to function without power for up to six hours, or have a power supply that cannot be interrupted
Can discharge to either a watercourse or drainage field
Drainage field – or soakaway - should be at least 15m from any building and 50m from a water supply, such as a well
Should be at least 7m away from any habitable parts of a building
Should be located within 30m of an access point for tank emptying
Should be at least 10m from a watercourse
Can only discharge to a drainage field, not a watercourse
Drainage field should be at least at least 15m from any building and 50m from a water supply, such as a well
No access roads, driveways or paved areas should be located within the area
For householders looking to install or replace an off-mains treatment system, the process can seem daunting, but decisions generally come down to five considerations: size of property; cost of purchase, installation and maintenance; environmental impact; landscaping and gardening potential; and risk of odour.
Sewage treatment systems are designed to minimise environmental impacts and water pollution by providing effective treatment of wastewater. Reducing pollutants like phosphorus and ammonia is crucial for protecting aquatic ecosystems and public health, as these substances can cause significant harm if released untreated.
Read more – Domestic sewage treatment plants: The dos and don’ts of everyday use
Sizing a package sewage treatment plant generally depends on a calculation based on the number of bedrooms in the property. This assumes an average of two people per bedroom, plus one additional person for the household. So, for a three-bedroom house, the population estimate (PE) is seven.
Package sewage treatment plant should be sized at least 1.5 to 2 times the predicted total daily flow, to handle peak flows and ensure efficient treatment.
British Water’s Best Practice Guide to Flows and Loads provides a table which allows the total daily sewage load from properties to be calculated and it is recommended that all designers use this table when sizing and designing a non-mains sewage treatment system.
Septic tank size is usually based on the predicted daily flow rate. Generally, septic tanks are sized larger than a sewage treatment plant. A common rule is to have a tank capacity of at least 2.5 to 3 times the daily flow rate. This allows for adequate settlement and digestion of solids.
Consider unusual water uses such as spa baths and multiple bathrooms or utility rooms, as these can increase the biological load on the system.
The calculated size provides the minimum requirement, so it is advisable to round-up to the nearest available size of package sewage treatment plant or septic tank, to ensure adequate capacity.
Read more – Do you need a bigger septic tank or sewage treatment plant for your home extension?
Ideal Septic Tank Replacement
One advantage of having an off-mains drainage and sewage treatment system is that you will not pay water company sewerage rates. However, you will need to have the solids collected in your package sewage treatment plant or septic tank removed regularly.
Desludging is a necessary maintenance task for sewage treatment plants to prevent sludge buildup and maintain efficiency. Regular desludging helps avoid operational issues and ensures the system continues to function effectively.
The price of the sewage treatment system and ongoing maintenance costs should be taken into consideration before you make an investment. Regular inspections and proper maintenance are essential for extending the lifespan and improving the efficiency of sewage treatment plants. Consider the whole-life cost of different systems – those with fewer mechanical moving parts and electrical connections are less likely to break down and will require less maintenance over their lifespan.
Maintenance requirements for domestic sewage treatment plants vary depending on the size, type, and age of the equipment. When selecting equipment, compare the length and scope of warranties for different systems.
Water companies play a vital role in upgrading infrastructure, monitoring environmental performance, and ensuring compliance with permits and regulations related to sewage treatment.
When undertaking an installation, costs can vary depending on the site location, quality of the ground, the model chosen, and how long installation takes. Where an existing system needs to be removed, this would also need to be factored into estimates from installation contractors.
Wastewater treatment systems eliminate the harmful microorganisms and pollutants in sewage that can cause illness and disease. Both package sewage treatment plants and septic tanks separate out many contaminants in the solids that are later removed by tanker.
The additional stages of treatment in a package sewage treatment plant means a higher quality of water is achieved before the treated effluent is discharged from the tank and enters the ground or watercourse. This gives greater protection to the environment, plants, wildlife, and aquatic life in rivers, streams, and other water bodies by reducing harmful pollutants such as phosphorus and ammonia. Maintaining high water quality in these water bodies is essential for public health and recreational activities.
There are tight regulations around discharging wastewater near watercourses and these are even stricter at Sites of Specific Scientific Interest (SSSIs). Sewage treatment plants must comply with strict environmental regulations to ensure that the treated water does not harm aquatic life or public health. The quality of treated sewage is monitored to ensure it meets environmental standards, helping to protect public health and ecosystems. Discharges from septic tanks directly to a watercourse are not permitted under current environmental regulations. In this scenario, septic tanks should be replaced with a package sewage treatment plant.
Read more – 9 reasons to install a domestic sewage treatment plant
Gardening over a package sewage treatment plants and drainage field with care can enhance a property aesthetically, while ensuring the all-important functioning of the wastewater system. For modern systems, odours are no longer an issue, and package sewage treatment plants can be located below-ground.
Because drainage fields are moist and nutrient rich, with careful landscaping and plant selection, it is possible to create a space that is both beautiful and functional.
Choosing the best off-mains wastewater treatment system for your site will give you peace of mind for many years to come. Here is a summary of key points:
If your property has off-mains drainage, you may require a package sewage treatment plant or a septic tank.
Planning rules and environmental regulations for package treatment plants and septic tanks vary, so check which option is best for your site.
If the property discharges to a watercourse, a package sewage treatment plant will be necessary.
Ensure your treatment system is sized to handle peak flows and loads at your property
Careful landscaping and garden planting can ensure that sewage treatment systems can meet the aesthetic expectations of today’s householders.
Odour is no longer an issue with modern systems with the correct installation and servicing of the plant.
